In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition. In LTE networks, Physical Downlink Control Channel (PDCCH) is used for downlink (DL) scheduling or uplink (UL) scheduling of Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) transmission. Typically, PDCCH can be configured to occupy the first one, two, or three OFDM symbols in a subframe. The DL/UL scheduling information carried by PDCCH is referred to as downlink control information (DCI).
The DCI format is a predefined format in which the downlink control information is formed and unicasted by a serving base station to each UE in PDCCH. The DCI format gives the UE details such as number of resource blocks, resource allocation type, modulation scheme, transport block, redundancy version, coding rate etc. Different DCI formats are defined in LTE to support different transmission schemes. In LTE, unicast DCI is supported, where a single DCI carries the scheduling information of DL/UL data for one specific UE only. For unicast DCI, each UE needs to blindly decode PDCCH over specified search spaces for a specific DCI format. Unicast DCI has its advantages and disadvantages. On the upside, the DCI overhead is proportional to the number of scheduled UEs within a scheduling time unit, e.g., subframe in LTE. On the downside, CRC bit is duplicated for each unicast DCI and it introduces high CRC overhead when there are multiple UEs scheduled in the same scheduling time unit. In addition, each unicast DCI usually has small information size (e.g., around 50 bits) and it cannot exploit sufficient channel coding gain. For example, Polar code, which is adopt in the next generation 5G system, has higher channel coding for the information size around 100 or larger bits. Such higher channel coding gain cannot be achieved for smaller DCI size.
A solution is sought to enhance DCI transmission efficiency.